(1) Field of the Invention
The present invention relates to an optical transmission apparatus and a control method therefor, and more particularly to an optical transmission apparatus and a control method therefor to be used in an optical transmission system which wavelength-division multiplexes a plurality of signal lights and transmits the multiplexed signal lights.
(2) Description of the Related Art
Wavelength-division multiplexing optical transmission for transmitting a plurality of signal lights differing in wavelength bundled into a single optical fiber is generally applied to optical communication systems with a view to increasing the communication capacity and reducing the system cost. In an actual system, in order to compensate for optical signal losses arising in the optical fiber, which serves as the transmission path between two distant points, an optical fiber amplifier is installed on the transmission path, and a plurality of signal lights differing in wavelength are collectively subjected to signal amplification without converting them into electric signals on the way of transmission.
The amplification rate (amplification gain) of an optical fiber amplifier is dependent on the wavelength of signal lights. For instance, in the case of an optical fiber amplifier for amplifying optical signals in a wavelength range of 1530 nm to 1560 nm, the amplification gain of signal lights in the vicinity of 1530 nm is greater than that of signal lights in the vicinity of 1560 nm. For this reason, when collectively amplifying a plurality of multiplexed signal lights differing in wavelength, the flattening of gains depending on the wavelength is required. For the purpose of such gain flattening, a dielectric multi-layered film filter or a gain flattening filter using fiber Bragg grating, for instance, is mounted within the optical fiber amplifier.
The wavelength-dependence of the amplification gain varies with the optical input power of the optical fiber amplifier. In other words, as the wavelength-dependence of the gain varies with ups and downs of the gain of the optical fiber amplifier, even an optical amplifier equipped with a gain flattening filter can achieve gain flattening only while the gain of the optical fiber amplifier is maintained at its designed level. For this reason, in an optical fiber amplifier, in order to flatten the gains of multiplexed signal lights of a plurality of wavelengths, automatic gain control is performed to keep the gain constant all the time by monitoring the powers of input and output lights.
Further in an optical transmission system, to cope with the limitation of the input dynamic range of a receiver, the nonlinear effect of the optical fiber and the like, automatic output level control is performed to keep the output signal light power of each wavelength constant, in parallel with the aforementioned automatic gain control. In an optical transmission system having no automatic output level control function, as a variation in losses on the transmission path leads to a variation in the power of input optical signals of an optical amplifier for signal relaying, and the power of output optical signals of the optical amplifier also varies, eventually the power of optical signals inputted to an optical transmission apparatus on the receiving side may vary to cause the input signal level of a receiver to deviate from the dynamic range.
In order to keep the power of optical signals to be inputted to the receiver constant, for instance, an optical attenuator may be inserted on the signal path of the optical fiber amplifier to adjust the attenuation factor according to the losses arising on the transmission path on the signal input side. Since the wavelength-dependence of the extent of attenuation in the optical attenuator is not affected by the optical input power, combination of an optical fiber amplifier and an optical attenuator makes possible both automatic gain control and automatic output level control.
Automatic gain control in an optical fiber amplifier can be achieved by, for instance, observing the powers of wavelength-division multiplexed signal lights on the input and output sides of the optical fiber amplifier, and controlling the pumping power of the amplifier so as to keep the power ratio (gain) between the input signal lights and the output signal lights constant all the time. Automatic output level control for each wavelength can be accomplished by, for instance, figuring out the total output optical power of the optical fiber amplifier from a predetermined number of multiplexed signal lights (the number of multiplexed wavelengths) and the output optical power of each wavelength, and controlling the extent of attenuation of the optical attenuator so as to bring the total output optical power of the optical fiber amplifier to a desired level.
However, the automatic output level control method described above involves a problem when a difference arises between the number of multiplexed wavelengths, which is the condition for figuring out the total output optical power, and the number of wavelengths of the multiplexed signal lights actually entered into the optical fiber amplifier. In an optical transmission system, the number of signal lights to be wavelength-division multiplexed (the number of wavelengths) into the optical fiber will change, for instance, when one of a plurality of transmitters coupled to the optical fiber runs into trouble or an optical fiber coupling the transmitter with the wavelength multiplexer comes off.
In this case, since each optical fiber amplifier cannot recognize the number of actually multiplexed wavelengths at that moment when the trouble has occurred, matching between the number of wavelengths, which is a precondition for the automatic output level control, and the number of wavelengths physically multiplexed on the optical fiber can not be achieved. Therefore, when optical signals of a certain wavelength becomes absent on account of maintenance work or trouble, if automatic output level control of the fiber amplifiers is performed targeting on a total output optical power calculated on the basis of a greater number of signal lights than that of signal lights actually multiplexed, the output optical power per signal light will become higher than the expected level, resulting in a problem that each signal light eventually reaches the receiver at an excessively high input signal level.
To address such problems, for instance, in the Proceedings of the 1996 Communications Society Conference of the Institute of Electronics, Information and Communication Engineers (IEICE), Lecture No. B1096 (Non-Patent Reference 1), there is proposed an optical amplifier output level control for WDM which performs control so as to bring the output power per signal light to a desired level by detecting the total signal light power outputted from the optical amplifier and the number of wavelengths accommodated in the system. Further, in the Japanese Unexamined Patent Application Laid-Open No. 2001-257646 (Patent Reference 1), there is proposed a method for extracting a supervisory signal light for monitor and control, known as a pilot or probe signal light, with a branching element provided on the output side of an optical amplifier and controlling the optical amplifier so as to keep the optical power of the probe signal light constant.
In the conventional optical amplifier, it is supposed that the velocity of loss variations on the transmission path is sufficiently slower than the control velocity of the optical amplifier while the transient response of signal power variations due to change in the number of wavelengths to be multiplexed is sufficiently faster than the control velocity of the optical amplifier, and the above two varying factors are discriminated from each other by the difference in the variation velocity of optical signal power observed with an optical transmission apparatus. Any change in the number of wavelengths is an event that occurs in an operation to alter the communication path linking the sending point and the receiving point for instance, and the varying velocity of the number of wavelengths is supposed to be not more than a few hundred μs. On the other hand, any variation in losses is an abnormal event that occurs, for instance, when the maintenance personnel of the optical transmission system pulls or catches the optical fiber, and the varying velocity of losses on the transmission path is supposed to be not less than a few ms.
To take note of this difference in varying velocity, it is able to determine whether any variation in signal power is due to a loss variation on the transmission path or due to a change in the number of wavelengths by setting a threshold of a frequency regarding the variation in total signal power to be detected from the multiplexed input signal lights of the optical amplifier and judging whether the velocity of variation in the total signal power due to the occurrence of a given event has exceeded the frequency threshold. It is also possible to determine the control mode of the optical amplifier to be executed in accordance with the identified cause of the total signal power variation. As the control mode of the optical amplifier, automatic output level control should be performed when a loss variation occurs and automatic gain control should be performed when a variation in the number of wavelengths occurs. No mismatching between the event that has actually occurred and the selected control mode is permissible.
However, by the above-described method to determine the nature of an event with reference to a frequency threshold, if a variation in the number of wavelengths occurs at a low velocity in or above the ms order, for instance, the variation in total signal power will not be perceived as being consequent on a variation in the number of wavelengths but will be erroneously attributed to a loss variation on the transmission path. In this case, automatic output level control having as its target value an output optical power of the optical amplifier is selected, resulting in a wrong control operation to increase the signal light power of each wavelength more than required. Furthermore, whereas two control modes including automatic output level control to compensate for variations in signal power due to loss variations and automatic gain control to compensate for variations in signal power due to variations in the number of wavelengths coexist in the optical amplifier, if it is difficult to identify the cause of variations as mentioned above, the two control modes may be executed at the same time with an adverse effect on signal quality even though one or the other type of variation causes has occurred.
The method proposed in Non-Patent Reference 1 to control the optical amplifier by detecting the number of multiplexed wavelengths involves the problem of permitting proper responses only to relatively slow loss variations on the transmission path or relatively fast variations in the number of wavelengths. Furthermore, since automatic output level control is always at work according to Non-Patent Reference 1, the optical amplifier may erroneously operate not only when an abnormal event has occurred but also in response to a normal operation by the system administration staff.
According to Patent Reference 1, as automatic output level control or automatic gain control of the optical amplifier is performed by taking note only of power variations in the probe signal light, there is no need to discriminate the causes of variations in signal light power unlike in Non-Patent Reference 1. Furthermore, since there is no particular restriction on the response time constant of the optical amplifier, fast loss variations on the transmission path at or below a few ms or slow variations in the number of wavelengths at or above a few ms can be coped with. According to Patent Reference 1, however, as the control of the optical amplifier is dependent on a specific probe signal light, there is a problem that, if any abnormality of the probe signal light arises for some reason such as a failure in the light source, the optical amplifier will become uncontrollable, inviting a trouble in the transmission of signal lights.